Research is in progress to determine the relationships between the structure and delayed neurotoxicity of a group of phosphonate, phosphonothioate and phosphoramidate esters related to leptophos (0-(2,5-dichloro-4-bromophenyl) 0-methyl phenylphosphonothioate. Comparisons of the effects of single and chronic oral doses are being made, and the pattern of enzyme inhibition is being evaluated. Related studies include the degradative fate of selected radiolabeled neurotoxic phosphonothioate esters in mice, chicks and laboratory model ecosystems.